This application claims the priority of German Patent 197 13 633.8, filed Apr. 2, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for removing NO from the exhaust gas of internal-combustion engines, particularly for use in a motor vehicle.
Increasingly strict emissions laws require development of more efficient exhaust gas aftertreatment systems for motor vehicles. For meeting future laws, several avenues are being explored for removing nitrogen from lean engine exhaust gases:
DENOX (NO.sub.x -removal) Catalysis
Using a reducing agent, such as ammonia or a hydrocarbon, nitrogen oxides are converted on a suitable catalyst to nitrogen and water. When a hydrocarbon is used, carbon dioxide is also produced. DENOX-catalysts developed so far have too little activity and are not selective enough. Also, for the most part, long-term stability of the catalyst in the exhaust gas is limited.
NO.sub.x -Storage Catalysts
This catalyst consists of two components. A NO.sub.x -storing material and a three-way catalyst comprise this aftertreatment system. In lean operation of an engine, nitrogen oxides are chemically bound as nitrate in the storage component. They are released in a regeneration phase under stoichiometric or rich engine operating phases and converted on the three-way catalyst to nitrogen. The NO.sub.x -storing material is susceptible to SO.sub.x degradation and has limited thermal stability. These factors currently prevent use of this type of catalyst in mass-produced vehicles.
SNR-Process (Selective NO.sub.x -Return)
Nitrogen oxides are stored on a suitable material in the exhaust gas. After a complete charging of the NO.sub.x storage device they are returned by way of a corresponding flap control in the exhaust routing via the intake air into the engine combustion chamber where reduction takes place during combustion. So far, an insufficient storage device charging capacity and limited life have prevented use of an SNR-process in mass-produced vehicles.
All of the above processes are still in the development stage and cannot be used in series-produced vehicles.
It is therefore an object of the invention to provide a process for NO-removal from engine exhaust gases without the above disadvantages, particularly lack of activity, lack of selectivity and lack of long-term stability.
This object is achieved by conducting NO-containing exhaust to a first electrode where NO is anodically oxidized to NO.sup.+, transporting NO.sup.+ through a solid electrolyte, and then cathodically reducing the NO.sup.+ to NO.
According to the invention, an NO.sup.+ -conducting electrochemical solid electrolyte is used. The electrolyte is arranged between two electrodes and withdraws NO from exhaust and transports the NO into an enrichment cell when a voltage is applied. NO on the exhaust gas side is anodically oxidized to NO.sup.+ and transported through the solid electrolyte. On the cathode, the nitrogen oxide cations are reduced to NO.
A cation-conducting solid is generally used as the solid electrolyte. NO.sup.+ - or Ag.sup.+ -exchanged beta aluminum oxide are mentioned as embodiments.
In contrast to known processes, the process of the invention has the advantage that nitrogen oxides can be removed from exhaust gas without using additional reducing agents.
The process according to the invention can be used for meeting future laws for removal of NO.sub.x from exhaust gases. It can be used particularly for removing nitrogen oxides in the exhaust gas of .lambda.=1-operated or lean-mixture operated motor vehicles.
By means of nitrogen oxides enriched on the cathode side, different processes can be carried out after the selective removal of NO.sub.x. Some of these are briefly described below:
Case A
Return of NO by way of intake air into the engine combustion chamber (according to the SNR-principle) (FIG. 2).
Case B
Conversion of NO under stoichiometric/rich exhaust gas conditions for lean-mix operated Otto engines (direct injection or intake pipe injection) (FIG. 3).
Case C
Conversion of NO by gaseous reducing agents (such as hydrocarbon, CO, H2) generated or carried along on board the motor vehicle (FIG. 4).
Case D
Catalytic reduction of NO by H.sub.2 as H.sup.+ produced from exhaust gas water transported through the solid electrolyte as H.sup.+ or through an additional H.sup.+ -conducting membrane (for example, a PEM Proton Exchange Membrane) (FIG. 5).
Case E
Cracking of NO on the cathode side on a catalyst while releasing N.sub.2 (gaseous) and transporting cracked oxygen through an O.sup.2- -ion conductor which is known (FIG. 6).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.